Model Diagnostic Discussion
NWS Weather Prediction Center College Park MD
153 PM EDT Sat May 11 2019

Valid May 11/1200 UTC thru May 15/0000 UTC

...Corrected to change header to 12Z NAM/GFS

...See NOUS42 KWNO (ADMNFD) for the status of the upper air
ingest...

12Z NAM/GFS Evaluation Including Preliminary Preferences and
Forecast Confidence
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...Overall Pattern Across the CONUS...
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An amplified upper level pattern will remain over the lower 48
through Tuesday with northern stream troughing approaching the
West Coast for Monday, with downstream ridging over the western
half of the nation and troughing over the eastern half of the
nation. In addition, a southern stream closed low, currently
offshore of the northern Baja Peninsula, will track into Texas for
Monday night. There are three main systems to speak of which will
have the greatest impacts on the lower 48 through Tuesday evening.

...Closed low moving through the Ohio Valley...
...Surface low development off of the East Coast...
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Preference: 12Z NAM/UKMET blend
Confidence: Slightly above average

An initial sfc-850 mb low will track through the upper Ohio
Valley, with the 00Z ECMWF noted as a stronger outlier. While the
ECMWF has been consistent with this feature, it has little support
outside of its ensembles. The 00Z ECMWF position appears
reasonable however. The 12Z GFS is slightly weaker than the
consensus, so overall, a 12Z NAM/UKMET blend should reduce the
strength of the 00Z ECMWF while increasing the 12Z GFS strength to
a favorable compromise.


...Closed low approaching West Texas on Monday, weakening to an
open wave by Tuesday morning...
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Preference: 12Z GFS/00Z ECMWF blend
Confidence: Average

Overall, model agreement with this feature is good except for the
00Z CMC which is a bit weaker with the mid-level low by Monday
evening. Differences develop due to convective handling in the
models, but for the purposes of this discussion focusing on mass
fields, a 00Z ECMWF/12Z GFS blend matches well with the latest
ensemble means.


...Upper trough/cold front approaching the Pacific Northwest...
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Preference: General model blend
Confidence: Average

There are mainly timing differences with this system, the 12Z
GFS/UKMET are faster while the 12Z NAM/00Z ECMWF/00Z CMC are a bit
slower. The 12Z UKMET differs perhaps the most with the 500 mb
pattern nearing the Pacific Northwest, but impacts to the coast do
not appear that significant with a general model blend resulting
in a solution close to the ensemble means which edge slightly to
the slower side of the latest deterministic spread.


Model trends at www.wpc.noaa.gov/html/model2.html
500 mb forecasts at www.wpc.noaa.gov/h5pref/h5pref.shtml

Otto